In a known fuel injection system of this type, the pump piston of the injection pump supplies fuel into the fuel reservoir until such time, toward the end of the compression stroke, as a valve has been pushed open by the fuel; the connection to a pressure line leading to a fuel injection nozzle is furnished by way of this valve. Only after this instant does the injection begin, its quantity being made up in part from the quantity already supplied into the reservoir. Although in this system the reservoir pressure is controllable, and despite a safety valve in the reservoir chamber, the fuel quantity supplied into the reservoir chamber for the most part does reach the point of injection (in other words, there are few losses), and the duration of injection does correspond more or less to the quantity of injection. However, the injection quantity itself is controlled by rotation of the pump piston which includes an oblique edge for determining the injection quantity; thus the intention of effecting fuel supply by way of the reservoir is to make the supply pressure substantially independent of the rpm. Since the pressure increases with an increasing supply quantity, because of the restoring force of the reservoir which increases in that case, the pressure control is effected in accordance with fuel quantity or load, instead of in accordance with rpm, as is conventional. However, engine manufacturers are requiring that at low rpm, for instance, in idling and at partial load, the injection duration should be relatively long, so as to attain quiet idling of the engine. The known apparatus described above is not capable of meeting this demand because the pressure control is made separate from the rpm; other conventional control means where only the rpm is a standard are equally unable to meet these demands.
In a different known fuel injection system, the prolongation of injection time during idling and partial load is effected by means of the outflow of a partial quantity, which is compensated for in terms of quantity because of the rpm governor. The quantity which has flowed out, however, is a lost quantity which is not utilized during the injection procedure.